1. Field of the Invention
The present invention relates to a bonding sheet for an electronic component, which is used when electrode terminals of the electronic component such as a semiconductor element are electrically connected to a conductive pattern formed on the substrate, and a method of bonding an electronic component, using the same.
In the course of recent technical developments, semiconductor integrated circuits have been miniaturized at a high density, some integrated circuit elements having 100 terminals or more. Consequently, a demand has arisen for the development of techniques for efficiently bonding the high-density integrated circuit element onto a substrate with conductive patterns. Among the techniques developed, a flip-chip method, in particular, which can simultaneously bond a large number of electrode terminals of a semiconductor integrated circuit to conductive patterns of a substrate, has received a great deal of attention.
The flip-chip method is a general term for face-down bonding wherein a semiconductor element is bonded to bonding pattern terminals formed on a substrate, while its active surface faces downward. This method can be mainly classified into a method wherein bonding is performed after metal projections (bumps) are formed on electrode terminals on a semiconductor element and a method wherein bonding is performed after bumps are formed on connecting terminals on a substrate with conductive patterns.
In order to form bumps, in a known method of the former one, a multilayered structure of Cr/Cu/Au or Ti/Ni/Au is formed by deposition, after which a Pb-Sn solder is plated thereover. In a known method of the latter one, a Pb-Sn solder is printed or plated. In either method, formation of bumps requires complicated processes, resulting in a low yield.
Using the flip-chip method, electrode terminals of the semiconductor element are bonded to connecting terminals on the substrate with conductive patterns, at a high temperature of 250.degree. to 330.degree. C. However, a solder used for the connecting terminals causes peeling, cracking, and the like, due to temperature distortion (arising from the difference in the termal expansion coefficient of the semiconductor element and that of the wiring substrate) when the solder is cooled from the high temperature upon melting and bonding to room temperature, resulting in poor reliability. Such a high temperature also degrades a device formed on the substrate. When a liquid crystal display is used for a substrate, an organic color filter is degraded. Since the heat-resistance temperature of a color filter is about 150.degree. C., its characteristics become degraded at the above-mentioned temperature.
Another known method of bonding the electrode terminals of a semiconductor element and connecting the terminals of a substrate or the like is the film carrier method. Using this method, a conductive layer on a resin film is bonded to electrode terminals on a semiconductor element by means of thermocompression bonding. More specifically, a bump formed of, for example, a Ti/Ni/Pd/Au laminated structure is formed on Al electrode terminals on a semiconductor element, and Sn is deposited on a Cu conductive layer on a film carrier. Heat at a temperature of 450.degree. to 500.degree. C. and a high pressure of 200 to 1000 kg/cm.sup.2 (20 to 100 g per 100 .mu.m.times.100 .mu.m terminal) are applied to both the bump and carrier, so that they are bonded by a gold-tin eutectic. However, as in the case of the flip-chip method, so also in the case of the film carrier method, temperature distortion occurs due to the high temperature. Also, since the above high pressure is applied to the semiconductor element, the element may be damaged.
Further, it is difficult to form electrode terminals having a uniform height due to an increase in the number of electrode terminals as described above. Therefore, a high pressure is locally generated between the bump and the connecting terminal (or electrode terminal), and the semiconductor element may be damaged.